The present invention relates to strips, wires or reshaped parts made of alloys of platinum and/or palladium as the base metals, particularly for use as electrodes in spark plugs. The invention also relates to processes for production of such strips, wires or reshaped parts.
The metals of the platinum group and their alloys have been used for many years as electrodes for spark plugs in combustion engines. Frequently, alloying additions of high-melting non-noble metals (e.g., W) and embedded oxides of the rare earth metals are used in order to minimize wear and tear due to spark erosion in use.
Materials which are particularly suitable for this application are alloys based on Pt with additions of Ir, Ru, W, Mo and/or Re. These alloying elements possess the common characteristic that they oxidize considerably more easily than platinum and form volatile oxides during oxidation.
Pt—Ir alloys hardened by oxide dispersion and other Pt alloys are known, which can be produced by the internal oxidation of non-noble metal components (see, for example, German published patent applications DE 197 14 365 A1 and DE 100 46 456 A1 and German Patent DE 197 58 724 C2). However, it has been found in the course of investigations that these materials have substantial disadvantages, if they are to be used as electrode materials in spark plugs.
Pt alloys containing both proportions of volatile oxide generators ≧5% by weight and proportions of incorporated oxides >0.1% by weight tend to form cracks during processing into thin strips or wires and into electrode tips, which typically have a diameter of <1 mm. In addition, the numerous annealing operations in an oxidizing atmosphere, which are necessary for the manufacture of strips or wire and reshaped parts, lead to undesirable losses of the alloying elements.